1. Field of the Invention
The present invention relates to a pneumatic tire in which a lot of projecting lines (ridges) are formed on an outer surface of a side wall portion.
2. Description of the Related Art
In a vulcanization molding of a tire, a pressure and a heat are applied for a fixed time to a green tire in a state in which an outer surface of a side wall portion is pressed against an inner peripheral surface of a mold. At this time, it is necessary to suitably let out air existing between the outer surface of the side wall portion and the inner peripheral surface of the mold, and if the air is trapped without smoothly letting out the air, a concave scratch called a light is generated due to a fault of a rubber flow, and there is a case that an appearance of the tire is deteriorated.
The mold used for the vulcanization molding of the tire is generally provided with a ventilation hole called a vent hole for letting out the air. However, since the vent holes are set in a dotted manner on a circumference of the side wall portion, there is a case that the air fails to escape, and the vent holes cannot securely prevent the light. Further, since a rubber projection called a spew is formed on the outer surface of the side wall portion, at the position where the vent hole is set, and a trace after cutting the spew remains like a stub, there is a risk that the appearance of the tire is deteriorated if the number of the vent holes is increased blindly.
As a result of making a study by the present inventor, it has been known that the light tends to be generated in a specific region, and a generating frequency is different. That is, it is known that the light in the side wall portion particularly tends to be generated in the following regions (1) to (3), the generating frequency is higher in the order of (1), (2) and (3). Further, it is known that a mode of the light is frequently generated along a tire circumferential direction, and it is important to inhibit such the light from being generated.
(1) periphery of interface between tread rubber and side rubber
(2) periphery of roll-up end of carcass ply
(3) periphery of leading end of bead filler
As mentioned in the items (1) to (3), there is a tendency that the light is generated in a peripheral region of an end of the member existing in the side wall portion. There can be considered that the vent hole is set aiming at a position of the member end, however, it is a fact that the member end tends to generate a dispersion on a process, and a light countermeasure only by the vent hole is not very effective. It is considered that the generating frequency of the light becomes higher in the order of (1), (2) and (3) mentioned above because the distance between the member end and the outer surface of the side wall portion becomes shorter in the order mentioned above, and the light is most frequently generated in the interface between the tread rubber and the side rubber which is exposed to the outer surface.
In Japanese Unexamined Patent Publication Nos. 2001-163018, 2004-17829, 11-198614, and 11-291722, there is described a pneumatic tire in which an outer surface of a side wall portion is provided with a lot of projecting lines (ridges) extending to a tire diametrical direction or a direction which is inclined from the tire diametrical direction, however, it is considered that they cannot sufficiently inhibit the light as mentioned above from being generated. In order to suppress the light in the side wall portion, it is very important to promote the rubber flow of the side rubber at a time of the vulcanization molding so as to make the air easily let out, however, the structure mentioned above cannot particularly promote the rubber flow.
Further, recessed strips corresponding to the projecting lines are provided in the inner peripheral surface of the mold which is pressed against the side wall portion, however, in the tire provided with a lot of protruding lines extending in the tire diametrical direction or the direction which is inclined from the tire diametrical direction, there is a problem that a man hour which is necessary for working the recessed strips is increased, and a mold working cost suddenly rises widely. Further, a tool tends to wear out in an early stage due to a lot of working amount, and there is a risk that a joint line appears in the projecting line so as to deteriorate the appearance of the tire, if the tool is replaced by a new one in the process of working the recessed stripe.